When neurologist Frances Ames began testing the effects of a single dose of cannabis sativa on a group of her medical colleagues who were, on the whole, “articulate and fairly stable people,” the onset of abnormal sensations “was always abrupt and immediate.” One was sustained hilarity. “The whole idea of the experiment,” Ames reported in 1958 in the Journal of Mental Science, “would suddenly seem enormously amusing.” Researchers have long been intrigued by the intoxicating effects of the world’s most popular illicit drug. Here’s everything you need to know about how pot affects your body and mind.

First, a brief bit of biology:

The body has an endogenous, or natural cannabinoid system. Endogenous cannabinoids play a role in the brain’s normal functioning, shuttling messages from one nerve cell to another. Not only the brain, but the spleen, uterus, testicles and other tissue have cannabinoid receptors. THC, the principal active component of weed, mimics a natural cannabinoid called anandamide, the “bliss molecule.” When smoked, THC quickly diffuses to the brain. “The consumption of cannabis causes a particular combination of relaxation and euphoria, commonly referred to as a ‘high’,” a committee of the U.S. National Academies of Sciences, Engineering and Medicine dryly noted last year in its exhaustive review of the health effects of cannabis. With very high levels of THC, things aren’t always so pretty.

Marijuana makes us laugh and scarf snacks

The academies report notes how, during acute cannabis intoxication, one’s “sociability” and appetite for sweet and fatty foods is heightened. “Munchies” may be driven by THC interfering with neurons in the brain normally involved in suppressing appetite, according to a study in Nature. When researchers injected mice with THC, the neurons responsible for shutting down eating were suddenly fired up. “It’s like pressing a car’s brakes and accelerating instead,” said lead author Tamas Horvath. The pathological laughter is harder to explain, although studies have suggested it may have to do with marijuana’s anti-depressant-like effects, as well as the drug’s ability to increase blood volume in the right frontal and left temporal lobes in the cerebral cortex, the brain areas thought associated with “mirth and laughter.”

The consumption of cannabis causes a particular combination of relaxation and euphoria

Pot increases the noise in your brain

In 2015, Yale School of Medicine researchers reported THC increased random “corticol noise” in the brains of healthy volunteers. Corticol noise is a kind of random activity in electrical circuits in the brain (think of the noise of a crowded room versus a single voice). This may explain THC’s sometimes psychosis-like effects. Some people, for reasons no one fully understands, experience a “robust syndrome” that mimics some aspects of schizophrenia, said Dr. Deepak Cyril D’Souza, a professor of psychiatry at Yale and the study’s senior author. They have symptoms of paranoia and hear voices. In the Yale study, volunteers given intravenous injections of THC showed an increase in neural noise on an EEG. The findings may add to other studies that suggest heavy pot use might precipitate or hasten schizophrenia, especially if used in early or mid-adolescence. The higher the use, the greater the risk.

Weed plays with your memory

In 2015, a team of Spanish researchers Spanish researchers reported heavy pot users appear more vulnerable to “memory distortions,” possibly due to decreased activity in the hippocampus, the region associated with memory. The effects were seen even though the pot users had not consumed marijuana in the month leading up to the study, “suggesting a long-lasting compromise of memory and cognitive control mechanisms involved in reality monitoring.” As UPI summarized it: “Stoners don’t make for good eyewitnesses.” The study also suggested that heavy pot users may remember things that never actually happened. The study involved 16 people who had used cannabis an average of 42,000 times over 21 years.

Pot makes you a slacker. Temporarily

Cannabis dampens neurons involved in motor response and excitation in various brain regions, “your kind of go-go response,” said Will Lawn, of University College London’s clinical pharmacology unit. He was the lead author of a study published two years ago where researchers asked volunteers to inhale pot vapour through a balloon on one occasion, and a placebo vapour on another. Then participants were given the option of completing a “low-effort” or “high-effort” task for cash. They could hit a space bar with the little finger of their non-dominant hand 30 times in 10 seconds for 50p (about 90 cents); or press it 100 times in 21 seconds to win up to two pounds (about $3.60). Volunteers on the placebo “pot” chose the high-effort option, on average, 50 per cent of the time vs. 42 per cent of those high on real pot. However, in a separate study, the researchers saw no differences in the willingness of long-term cannabis users to work for money when they weren’t stoned. In other words, cannabis use does not effect motivation when people are sober, Lawn said. Others report that long-term use of marijuana is associated with cognitive impairment, particularly in learning and remembering new things — but maybe not so much with older people. A study last year found that a low dose of THC reversed age-related declines in brain function in old mice.

It’s not clear why marijuana causes compulsive laughter.Getty Images

Where did the time go?

Even low doses of THC can make time seem to slow down. The effect disappears within a few hours, and it appears to be blunted in chronic (two to three times a week or more) users. One 2013 study found that even exceedingly low doses of THC can increase people’s “internal clock speed.” In that study, THC was administered intravenously to 44 people. Regardless of dose, volunteers overestimated how much time had passed, though frequent users showed no differences. Ames, the South African neuroscientist and psychiatrist, noted that, when stoned, her subjects reported that events that occurred immediately after each other “seemed separated by an eternity of time.” There’s no time centre in the brain, per se, said Dalhousie University professor of psychiatry Phil Tibbo. “But cannabis does effect some of those cognitive domains like attention, concentration, memory. That’s probably what’s influencing and resulting in, for some people, feeling time’s a bit different.”

Marijuana makes babies trickier

Heavy pot use might lower a man’s reproductive potential by affecting the morphology — size and shape — of his sperm. A study published in the journal Human Reproduction found males under 30 with less than four-per-cent normal sperm were nearly twice as likely to have used cannabis in the last three months. No similar associations were found with BMI, type of underwear, smoking, alcohol consumption or having a history of the mumps. Sperm with morphology issues are generally lousy swimmers, crawling and crashing head-on into the walls of the female reproductive tract in their frantic swim to fertilize an egg. Chronic cannabis use has also been linked to decreased libido in men. For women, some research suggests cannabis suppresses ovulation. It’s also been linked to early pregnancy loss and low-birthweight babies if used at the time of conception, or during pregnancy. Emerging animal data also suggests using pot during pregnancy impairs fetal brain development.

The good and the bad:

According to the academies of sciences review, the best evidence suggests smoking weed doesn’t increase the risk for lung, head or neck cancers in adults, though there is “modest” evidence linking it to a subtype of testicular cancer. The evidence is “unclear” as to whether cannabis increases the risk of heart attack or stroke, but there’s some evidence it has anti-inflammatory effects. Using pot before driving increases the risk of being in a crash but it doesn’t appear to increase the likelihood of developing depression, anxiety or post-traumatic stress disorder.

When do you know things are becoming problematic?

According to the fifth and latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), psychiatry’s official catalogue of mental illness, individuals with “cannabis use disorder” may spend many hours a day under the influence of pot. Other criteria include spending a great deal of time obtaining, using or recovering from the effects of cannabis and a need for markedly increased amounts of the drug to achieve the desired effects. New to DSM-5 is “cannabis withdrawal,” caused by the abrupt cessation in heavy daily, or near daily, pot use. Symptoms can include irritability, anger or aggression, restlessness, sweating, fever, chills and hypersomnia.

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